docs: def Yashar(x, rhol, rhog, mul, mug, m, D, g=g):
      r'''Calculates void fraction in two-phase flow according to the model of 
      [1]_ also given in [2]_ and [3]_.
      
      .. math::
          \alpha = \left[1 + \frac{1}{Ft} + X_{tt}\right]^{-0.321}
          
      .. math::
          Ft = \left[\frac{G_{tp}^2 x^3}{(1-x)\rho_g^2gD}\right]^{0.5}
                  
      Parameters
      ----------
      x : float
          Quality at the specific tube interval []
      rhol : float
          Density of the liquid [kg/m^3]
      rhog : float
          Density of the gas [kg/m^3]
      mul : float
          Viscosity of liquid [Pa*s]
      mug : float
          Viscosity of gas [Pa*s]
      m : float
          Mass flow rate of both phases, [kg/s]
      D : float
          Diameter of the channel, [m]
      g : float, optional
          Acceleration due to gravity, [m/s^2]
  
      Returns
      -------
      alpha : float
          Void fraction (area of gas / total area of channel), [-]
  
      Notes
      -----
      [1]_ has been reviewed; both [2]_ and [3]_ give it correctly.
          
      Examples
      --------
      >>> Yashar(.4, 800, 2.5, 1E-3, 1E-5, m=1, D=0.3)
      0.7934893185789146
  
      References
      ----------
      .. [1] Yashar, D. A., M. J. Wilson, H. R. Kopke, D. M. Graham, J. C. Chato,
         and T. A. Newell. "An Investigation of Refrigerant Void Fraction in 
         Horizontal, Microfin Tubes." HVAC&R Research 7, no. 1 (January 1, 2001):
         67-82. doi:10.1080/10789669.2001.10391430. 
      .. [2] Xu, Yu, and Xiande Fang. "Correlations of Void Fraction for Two-
         Phase Refrigerant Flow in Pipes." Applied Thermal Engineering 64, no. 
         1-2 (March 2014): 242–51. doi:10.1016/j.applthermaleng.2013.12.032. 
      .. [3] Dalkilic, A. S., S. Laohalertdecha, and S. Wongwises. "Effect of 
         Void Fraction Models on the Two-Phase Friction Factor of R134a during 
         Condensation in Vertical Downward Flow in a Smooth Tube." International 
         Communications in Heat and Mass Transfer 35, no. 8 (October 2008): 
         921-27. doi:10.1016/j.icheatmasstransfer.2008.04.001.
      '''
      G = m/(pi/4*D**2)
      Ft = (G**2*x**3/((1-x)*rhog**2*g*D))**0.5
      Xtt = Lockhart_Martinelli_Xtt(x, rhol, rhog, mul, mug)
      return (1 + 1./Ft + Xtt)**-0.321